Apparatus for continuous casting

ABSTRACT

Apparatus is described for continuously casting an ingot wherein a cooled plug is repeatedly brought into contact with the top of the ingot during the continuous casting process to remove heat from the central region of the ingot.

United States Patent 1191 1111 3,709,284 Hunt 1 Jan. 9, 1973 [54]APPARATUS FOR CONTINUOUS [56] References Cited CASTING UNITED STATESPATENTS [75 1 Inventor: Charles d Mmaga 3,620,686 11/1971 Pfann ..l64/82x [73] Assgnee FOREIGN PATENTS 0R APPLICATIONS [22] 1971 188,803 11/1922Great Britain ..l64/85 211 Appl 200 625 1,107,900 5/1961 Germany..l64/82 258,977 11/1963 Australia ..164/89 Related U.S. ApplicationData Primary Examiner--Robert D. Baldwin [62] Division of Ser. No.44,130, June 8, 1970, Pat. No. Assistant E Roethe] 3,653,116Attorney-Fitch, Even, Tabin & Luedeka [52] U.S. Cl. ..l64/28l, 164/120,164/321 [57] ABSTRACT Int. Cl. Apparatus i described for continuouslycasting an [58] Field of Search....164/82, 85, 89,120, 273, 281,

ingot wherein a cooled plug is repeatedly brought into contact with thetop of the ingot during the continuous casting process to remove heatfrom the central region of the ingot.

5 Claims, N0 Drawings I or 56 77 I 82 6 j I 47 ELECTRIC 2. CONTROL assvsrcu a 21 as 31 7 44 (37 2e 4 ll 1 39 35 1 42 v I e2 Es: m 3 F D 1'PATENIEDJM 9 ms SHEET 2 OF 2 3,709,284

Z l F LZUIUUQJIQQ APPARATUS FOR CONTINUOUS CASTING This application is adivisional application of application Ser. No. 44,130, filed June 8,1970, now US. Pat. No. 3,658,116, and assigned to the assignee of thepresent invention.

This invention relates to the continuous casting of metal ingots and,more particularly, to a method and apparatus for continuously casting aningot wherein problems of segregation are minimized.

The process of continuously casting metal ingots is well known andwidely used in industry. Generally, the continuous casting processemploys a casting mold having a cooled outer wall and a movable bottomor plug. Molten metal -is poured into the open top of the mold and, asthe metal solidifies in the mold, it is drawn downwardly by the plugwhile at the same time additional molten metal is poured into the moldat the top. The mold is of sufficient length and the casting rate issufficiently slow to allow the outer annular region of the ingot tosolidify enough to prevent breakout or enlargement due to the pressureof the internal metal which may still be molten. The process iscontinued until the supply of molten metal is depleted or otherwise cutoff and the solidified ingot is then drawn downward until it iscompletely removed from the mold.

For certain materials, such as highly alloyed tool steels, bearing racesteels, and many high temperature service nickel base alloys,segregation problems may occur in continuously cast ingots. For example,serious segregation of carbides or carbonitrides may develop and show upas relatively large nodules (freckling), continuous films, or elongatedrods, all of which are difficult to break up and redistribute intosolution by forging or heat treatment in subsequent stages of thematerial processing. Another type of segregation, known as coring, maydevelop between the majority elements of the alloy. This problem ismanifested by a difference in concentration of the majority elementswithin each crystal or grain of the ingot between the lastintracrystalline areas to freeze and the first intracrystalline areas tofreeze. In other words, as a crystal or grain freezes, dendrites growinward from the grain boundary and enlarge. Segregation may occur alongthe dendrite-liquid interface resulting in coring. Prohibitively largeannealing times may be required to diffuse the majority elements to amore uniform distribution within the grains.

A third type of segregation problem may occur, which is similar tocoring but which occurs over distances of several inches rather thanwithin the intracrystalline structures. This third problem may occurwith the majority elements, but may also occur with elements such assulphur or phosphorus, and results in a variation in alloy constituentsor in phosphor or sulphur content in different regions of the ingot.Such a problem typically occurs where certain regions of the ingot,usually the center, cool very slowly with respect to other regions.

In order to avoid segregation problems such as are described above incontinuously cast ingots, the solidification rate may be maintained at asufficiently high level. The solidification rate may be controlled bycontrolling the casting rate or the cooling rate of the ingot. Highersolidification rates and hence a fairly rapidly moving liquid-solidinterface during the liquid solid transformation will minimize thedevelopment of carbide and carbonitride segregation by shortening thetime available for nucleation and buildup of higher melting pointsegregates. Moreover, a high solidification rate with accompanying highliquid-solid interface movement produces relatively small secondarydendrite arm spacing (of the order 'of 1 to 10 microns). Diffusion ofthe alloy constituents is thereby possible over relatively smalldistances and therefore can take place in correspondingly shorter timesat appropriate annealing temperatures. In addition, alloy compositiongradients existing between dendrite arms in interdentritic spaces aremaintained relatively small with small secondary arm spacing because thefaster solidification time allows less time for solute redistribution.This again enables a shorter time at the annealing temperature. Finally,the rapidly moving liquid-solid interface with its consequently shortersolidifying zone minimizes the production of compositional variation inmajority elements over large distances.

In order to maintain a high solidification rate and thereby maintain arapid liquid-solid interface movement during the liquid-solidtransformation, appropriate cooling may be provided for the ingot as itis drawn downwardly below the mold. Systems for providing such coolingmay employ water sprays, baths of molten salts, or other appropriatemeans to increase the solidification rate by providing modes of heattransfer in and below the ingot mold in addition to the radiation mode.Where, however, a large diameter ingot is economically desirable toprovide a higher cast ing rate, or where additional modes of heattransfer are inconvenient, such as is often the case in continuouscasting processes in vacuum, heat extraction through the ingot byconducu'on into the mold and heat extraction from the ingot to theregion below themold, is relatively constant and non-controllable, atleast in the central region of the ingot. I

To avoid segregation problems under the circumstances outlined above,the ingot may be cast at a rate which is sufficiently slow. To do this,the heat input to the ingot from molten material flowing into the top ofthe mold is made sufficiently less than the level of the 1 relativelyconstant heat extraction from the ingot that a solidification rate whichmaintains segregation within tolerable limits is achieved. Whereproduction requirements dictate high casting rates, however, a very deepmolten pool may result in the ingot, especially one of large diameter,with resultant slow solidification in the central region of the ingotand a subsequent segregation problem.

It is therefore an object of the present invention to provide animproved method and apparatus for continuous casting.

Another object of the invention is to provide a method and apparatus forcontinuous casting which alleviates segregation problems.

It is another object of the invention to provide an improved method andapparatus for removing heat'from continuously cast ingots during thecasting process.

Other objects of the invention will become apparent to those skilled inthe art from the following description, taken in connection with theaccompanying drawings wherein:

FIG. 1 is a sectional view illustrating apparatus constructed inaccordance with the invention;

FIGS. 2 and 3 are partial sectional views illustrating different stepsin the operation of the apparatus of the invention;

FIG. 4 is a circuit diagram of a hydraulic control system which may beused in the invention; and

FIG. 5 is a graph illustrating operating characteristics of theapparatus of the invention.

Very generally in practicing the invention, molten metal is introduceinto a cooled continuous casting mold 11 and the cast ingot 12 is pulleddownwardly through the mold. The molten pool at the top of the ingot isrepeatedly contacted, at the central region thereof, with a cooled plug13 to remove heat from the central region of the ingot.

In practicing the method of the invention, higher casting rates areachieved by removing heat from the central region of the ingot at thetop thereof. This is done by repeatedly contacting the molten pool atthe top of the ingot in the central region by the cooled plug. Thediameter of the region contacted by the cooled plug is preferably notless that about 8 inches smaller than the diameter of the ingot (or thedistance across the flats in the case of a round comer square ingot).This ensures that the depth of the annular molten pool between thesolidified central section and the solidified outer wall of the ingotwill not be so great that solidification rates in the middle of the poolare slower than those desired. Preferably, the area of the plug whichcontacts the ingot is of sufficient size as to extend completely out tothe area where rapid freezing occurs due to cooling from the mold walls.This ensures that the depth of the molten ,pool will be substantiallyuniform across the entire cross section of the ingot. If the plug is toosmall and too little heat is removed, a central post forms in thecentral region of the ingot which is undercut by the superheat providedby the added molten material. Once the central post is undercut, the topof the solidified post will pull away with the plug when the plug isremoved. On the other hand, if too much heat is removed, cold shuts willresult on the outer surface of the ingot and moreover the new metalbeing added at the top of the ingot may make an unsatisfactory weld tothe existing solidified metal.

As mentioned before, it is preferred that the size of the plug besufficient to contact the ingot in the central region over an area whichextends all the wa'y out to where rapid solidification of moltenmaterial occurs due to cooling from the mold walls. Typically, this iswithin one or 2 inches from the inner surface of the mold. The plug isleft in contact with the ingot long enough to freeze or solidify most ofthe metal under the plug and is maintained out of contact with the ingotfor a period of time long enough to allow additional molten metal toflow into the region which has just solidified. The actual optimum timesmay be determined empirically depending upon the various heat transfercharacteristics of the system and the characteristics of the particularmetal being cast. Nevertheless, in-contact times for many metalstypically run in the range of about 3 to seconds for best results, andtypical out of contact times run in the range of about 2 to 6 seconds.Ideally, the system operation should be balanced so that the incontacttime is the maximum attainable and the out of contact time is onlysufficient to let a thin layer of new molten metal run into thecontacted region. Generally speaking, the thinner the laminates oflayers of molten metal which are contacted with each repetition, the

finer the metal grains and the less the segregation problems. It ispreferable that the depth of each lamination or each new layer of moltenmetal not exceed 36 inch when spread out due to contact of the cooledplug.

To effect the desired degree of heat removal, the plug is brought intocontact with the ingot with a substantial amount of force. This ensuresthat adequate heat transfer will take place and, moreover, efiects asubstantial amount of hot working of the, partially solidified material.This pushes voids in the material closed, and mechanically feeds thenatural shrinkage cavities around dendrites where the normal hydrostaticpressure of the melt is inadequate. The working also interrupts theformation of nucleated carbides or car-' bonitrides. Moreover, the morerapid cooling which is effected causes fast moving of the liquid-solidinterface so that a deep molten pool is not produced, thereby keepingthe freezing zone relatively small. 'Thus, segregation resulting fromtheiparticular kinetics of solidification of a given material is avoidedby the method of the invention by producing conditions in which anytendency toward nucleation is interrupted and broken up and in which thefreezing time is vary rapid and thefreezingzone very small. In otherwords, there is accomplished high heat removal from a relatively smallvolume.

The method of the invention may be more fully understood in connectionwith the apparatus of the invention and by referral to the accompanyingdrawings. The invention is illustrated therein in connection withcontinuous casting in'a vacuum system, for example, such as is shown anddescribed in U. S. Pat. No. 3,343,828 assigned to the present assignee.The invention is applicable, however, to other castingfsystems includingthose operating at atmospheric pressure.

The continuous casting mold 11 in FIG. 1 is provided I not shown, isattached to a suitable puller or plug, not

shown, to be drawn downwardly through the mold -l 1. Attachment to thepuller is usually effected by' allowing the molten metal to solidifyaround a dovetail type connection on the plug when the casting isstarted, asis known in the art. The illustrated ingot is cylindrical,however, other shapes may also be cast in accordance with the invention,one common ingot shape being th so-called round comer square ingot. I I

The plug 13 as illustrated has a conical lower surface 16. The plug 13is suitably secured to the lower end of a ram 18. The upper end of theram 18 comprises the The hydraulic cylinder 21 and the bridge 27 arecontained within a vacuum tight enclosure 32. The enclosure 32 ismounted over an opening 33 in the upper wall of the main vacuum tank 34.A launder 36 is provided within the tank 34 for conveying molten metalinto the mold from elsewhere in the vacuum system. The molten metal maybe poured into the ingot intermittently or continuously. The enclosure32 has an opening 37 therein and is mounted over the top of the vacuumtank about the mold with the opening 37 aligned with the opening 33 inthe tank. An annular vacuum seal 38 and a gate valve 39 are positionedbetween the enclosure 32 and the top of the main vacuum tank 34. Thegate valve 39 includes a main housing 41 and a slidable seal plate 42which is retractable to the position shown when the enclosure 32 is inplace and is pumped down, and which is movable to a closed positionwithin the valve housing 41 when it is desired to retract the ram andremove the apparatus for repair or replacement.

In order to be able to retract the ram and the plug 13 attached theretofrom the main vacuum tank 34 through the opening 33 therein, the bridge27 is supported for vertical movement. Four vertical screws 43, two ofwhich are shown in FIG. 1, are supported on the floor of the enclosure32 by bearings 44. The upper ends of the screws 43 are journalled inbearings 46 supported on brackets 47 which are mounted to the sidewallsof the vacuum tight enclosure 32. The bridge 27 is provided with fourinternally threaded collars 48, one for each of the screws 43. Thescrews 43 are in threaded engagement with the collars 48 and passthrough the bridge 27 through suitable openings, not shown, provided inthe bridge. The four screws 43 are rotated simultaneously by meansdescribed below to cause the bridge to move upwardly and downwardlywithin the enclosure 32. In FIG. 1, the bridge is shown in its lowermostposition, which is the position in which the apparatus is normallyoperated. When it is desired to remove the ram 18 and plug 13 from thevacuum chamber, the screws 43 are rotated to move the carriage of bridge27 to its uppermost position, with the upper plate 28 of the bridge justbeneath the brackets 47. In this position, the ram 18 and the plug 13may be moved to a position where the plug 13 clears the gate 42 in orderthat the gate may be closed and the apparatus of the invention removedfrom the position atop the vacuum tank 34.

In order to rotate the screws 43 simultaneously, a chain drive system isprovided. A spindle 51 at the upper end of each of the screws 47 passesthrough the associated bearing 46 and has a sprocket 52 mounted thereon.A chain 53 passes around the sprockets 52 and, through suitable idlersprockets, not shown, passes around a drive sprocket 54. The drivesprocket 54 is positioned in a sub-chamber 56 provided in the wall ofthe enclosure 32 and is driven by a drive shaft 57 which passes into thesub-chamber 56 through a suitable vacuum seal, not illustrated. Theshaft 57 is driven by an electric motor 58.

A hydraulic control system 61 is coupled by suitable hydraulic piping 62and 63 and flexible hoses 64 and 66 to the upper and lower chambers 24and 26, respectively. The control system 61 may comprise any suitablearrangement of solenoid operated valves and associated hydrauliccircuitry for exhausting the upper chamber 24 and inletting fluid to thelower chamber 26 when the ram is to be raised, and for exhausting thelower chamber and inletting fluid to the upper chamber when the ram isto be lowered. One type of control system is described subsequently inthis specification.

As previously mentioned, the invention has particular advantage inconnection with continuous casting in vacuum where the particular natureof the vacuum casting process precludes the use of well known expedientsfor effecting rapid solidification of the ingot. The vacuum tank 34 maycompletely enclose the unillustrated means for pulling the ingotdownwardly through the mold 11, but it is preferable that the tankterminate at the mold. In the latter case, the ingot may be pulledthrough a suitable vacuum valve, not shown, or may be pulled into avacuum tank, not shown, spe-. cially designed for receiving the ingot,such tank being releasably attachable to the main vacuum tank under themold 11.

Control over the movement of the ram 18 and hence the position of theplug 13 is provided by an electric control system 67. The electriccontrol system 67 may be of any suitable design to provide signals foroperating appropriate valves in the hydraulic control system 61 to causethe piston 23 to move upwardly or downwardly or stop within the housing22. Suitable circuitry, incorporating such elements as relays andswitches for controlling the valves in the hydraulic control system 61,may be easily designed by a person skilled in the art from thedescription of the operation of the apparatus setout in detail below.Accordingly, details of electric control system 67 will not be describedherein.

The upper end of the piston rod 19 is provided with a collar 71 to whichan arm 72 is attached extending horizontally therefrom. A rod 73 extendsdownwardly from the arm 72 into the space adjacent the hydrauliccylinder 21. A bracket 74 extends horizontally outward from the top ofthe hydraulic cylinder 21 and is provided with an opening thereinthrough which the rod 73 passes, thereby steadying and guiding the rod73 such that it remains in vertical relation with the piston rod 19 asthe piston rod moves vertically upward and downward. The lower end ofthe rod 73 is provided with an actuator plate 76. The actuator platethus moves upwardly and downwardly; in correlation with the movement ofthe piston 23.

Upward movement of the ram 18 is limited by providing a suitableelectrical signal to the electric control system to operatetheappropn'ate valves in the hydraulic control system 61. The electricalsignal for the electric control system 67 is caused by the actuation ofan upper limit switch 77 which is supported on a bracket 78 extendingdownwardly from one of the bearing support brackets 47 The actuatorplate 76 on the lower end of the rod 73 engages the switch 77 as thepiston 23 approaches the upper end of the hydraulic cylinder. Once theswitch 77 is actuated, the switch 77 being connected to the electriccontrol system 67 by suitable means not illustrated, the electriccontrol system provides a suitable signal to the hydraulic controlsystem 61 to stop the movement of the piston by actuating appropriatevalves.

In addition to the limit switch 77, the bracket 78 also supports afurther limit switch 79. The limit switch 79 is positioned to be engagedby the actuator plate 76 at a predetermined position in the downwardmovement of the piston 23. The predetermined position is selected suchthat it is just prior to the time the plug 13 engages the moltenmaterial at the top of the ingot 12. As will be explained in greaterdetail below, the signal provided by actuation of the switch 79, whichis connected to the electric control system 67, causes the hydrauliccontrol system 61 to operate in a manner which slows the downwardmovement of the ram 18. By doing so, movement of the ram 18 downwardlymay be initially fast, but may be slower prior to contact in order toavoid splashing of the molten material in the pool when the plug 13makes contact.

For the purpose of limiting the lower extent of travel of the ram 18, apressure sensor, described below, is provided in the hydraulic controlsystem 61 for sensing the pressure in the chamber 24 of the hydrauliccylinder 21. As will be explained, this pressure sensor provides anappropriate output signal when the pressure in the chamber 24 exceeds apredetermined level which is selected on considerations explained below.The pressure in the chamber 24 rises, as will be explained below, whenthe plug 13 encounters resistance in its downward movement due tocontact with solidifying material at the top of the ingot l2.

In operating the apparatus of the invention, the ram 18 is lowered untilthe plug 13 is partially immersed in the center of the molten portion ofthe ingot 12, such molten portion being indicated at 81. The surface 16of the plug thereby provides a central heat sink surface whichsolidifies the central portion of the ingot. This is in addition to theannular solidification resulting from the loss of heat into the cooledmold 11. The result is the formation of a liquid-solid interface 82having the appearance illustrated. Were it not for heat removal in thecentral region of the ingot, the liquid-solid interface would form agenerally parabolic outline part of which is indicated by the dottedline 83. The parabolic interface would extend very deeply into the ingotas shown and results in a very slow cooling rate in the central regionof the ingot. This slow cooling rate may result in segregation problems,as described above, and may under some extreme conditions become so deepas to require an excessive length in the mold walls in order to preventthe thin-walled partially solidified ingot from expanding or rupturingdue to insufficient strength to contain the pressure of the molten metaltherein.

In accordance with the invention, the ram 18 is reciprocated in orderthat the plug 13 is alternately plunged into the central region of themolten top of the ingot and is then withdrawn to allow molten metal tofill the cavity which it leaves. By selecting suitable operatingparameters such as the ingot withdrawal rate, the temperature of themolten metal, the time in which the plug 13 is immersed, and the time inwhich the plug 13 is withdrawn, it is possible to achieve uniformsolidification of the central region of the ingot. The thus solidifiedcentral region is free of voids and possesses a much finer grain sizethan in the surrounding annulus. This is due to the hot working whichoccurs when the cone contracts partially solidified metal as will beexplained.

In continuous casting carried out in high vacuum, it is preferred thatthe top of the ingot be heated (hot topped) by means of an electron beamor beams in order to eliminate the possibility of the formation of pipesor discontinuities in the casting. The beams are directed against thetop of the molten material'when the plug 13 is not in contact. When theplug is in contact, the beams may be allowed to impinge on part or allof the annular region of the ingot surrounding the plug in order toensure that molten metal will flow into the cavity left by the plug. Ifdesired, a tubular shield (not shown) may be placed around the plug andlower end of the ram 18 to prevent impingement of the electron beamsthereon. In this way, the heat removal characteristics are improved,since the electron beams do not strike the plug or ram directly.

Referring to FIG. 2, the plug 13 is shown in its inserted position. Themolten material in the cavity is spread out in a thin layer (a) over thetop of the ingot due to the action of the plug (shown exaggerated inthickness for clarity), and a slight rise in the levelof the liquid atthe mold occurs. This presents little problem, however, since plugswhich are immersed several inches in the molten material will cause arise which is typically less than 1/10 inch.

It may be seen from FIG. 2 that, when the plug 13 is immersed in themolten material 81 at the top of the ingot 12, heat is removed from thecentral region 84 by both the plug and the solid ingot. A shrinkagecavity may be produced along a line between the top surface of thesolidified ingot and the lower surface of the plug, but the cavity isforced closed and welded shut by the pressure of the plug. A ring (b) ofmolten material also forms between the plug and the mold, extendingdownwardly a distance which is slightly greater than the lowest levelreached by the apex of the plug. Preferably, the plug is forceddownwardly to provide a substantial degree of hot working. By doing so,it is also more readily possible to withdraw the plug without havingmetal adhere to its outer surface and thus produce heat of the moltenmetal in the ring (b), or the hot additional molten metal being addedfrom the tundish 36, or all of these cause the annular dam (c) to'rnelt.Molten metal then runs into the cavity and fills the cavity as shown inFIG. I. The lower portion of the molten pool may be partially-solidifiedas shown in FIG. 1 at (d) du'e to heat transfer into the solidifiedingot. The plug is then returned into engagement with the centralportion of the molten pool as shown in FIG. 2 again causingsolidification of the layer of molten metal between the plug and thesolidified ingot. Several sectioned layers are shown in the drawings,exaggerated in thickness for clarity, at 86. These layers represent thesuccessive layers which are squeezed or spread out and solidify uponeach stroke of the ram.

The shape of the plug 13 and in particular the lower surface 16 thereofmay be of any suitable form-depending upon the particular'casting systembeing used and the particular material being cast. In the illustratedembodiment, the surface 16 consists of a cone having a 150 includedangle. Other forms may comprise a conical surface having a 90 includedangle, a 120 included angle, a spherical surface, a flat surface withcylindrical sides, or a waffle iron type surface. The plug may becomprised of copper and may be utilized in its bare form or may beplated with a material such as chromium. Coolant passages, not shown,are provided internally of the plug.

Referring now to FIG. 4, a circuit diagram of the hydraulic controlsystem 61 may be seen. Included in the system is a four-port valve 87having ports 88, 89, 90, and 91. Depending upon the particularoperational condition of the valve 87, the various ports may be coupledto each other as shown by the direction of the arrows within therectangle. Thus, one condition of the valve connects the port 89 withthe port 88, while another condition of the valve connects the port 89with the port 90 and connects the port 88 with the port 91. In addition,the valve may be operated to a neutral or off condition in which all ofthe ports are blocked. A commercially available valve capable ofperforming in this manner is known as a four-way directional valveavailable from Vickers Corporation, Valve No. F3- EGS4-O4OC-20. The port89 is connected through a line 92 to a source 93 of pressurizedhydraulic fluid. The port 90 is connected through a check valve 94 tothe lower chamber 26 of the hydraulic cylinder 21. The port 88 isconnected directly to the upper chamber 24 of the hydraulic cylinder 21.A relief valve 96 is coupled across the ports 88 and 90 and a reliefvalve 97 is coupled across the ports 88 and 91. The port 91 is connectedto an exhaust region 95.

A second four-port valve 98 is provided in the system and includes fourports 99, 100, 101, and 102. The valve 98 is smaller than the valve 87for providing a lower rate of flow through the valve. The valve 98 iscapable of operating in the manner indicated by the arrows within therectangle such that the port 100 may be connected to the port 102. Inaddition, the valve is operable to a neutral or off position wherein allports are closed. The port 102 is coupled to the exhaust region 95 andthe port 100 is coupled through a flow regulator valve 103, and apressure reducing valve 104 to the source 93 of pressurized hydraulicfluid. The

port 99 is coupled to the upper chamber 24 of the hydraulic cylinder 21and the port 101 is coupled to the lower chamber 26 of the hydrauliccylinder 21. A suitable valve for the valve 98 is a miniaturedirectional valve sold by Vickers Corporation under Valve No. F3-DlL-2C-20.

In operating the hydraulic system 61, the electric control system 67changes the condition of the valves 87 and 98 in the proper order toeffect the operating sequence desired below. In order to lower thepiston 23 and thus lower the ram 18 (FIG. 1), the valve 98 is operatedto the neutral or off condition and the valve 87 is operated to acondition wherein the port 89 is connected to the port 88. In thiscondition, hydraulic fluid from the source 93 flows through the line 92,through the ports 89 and 88 and into the upper chamber 24 of thehydraulic cylinder. Pressure in the lower chamber 26 of the hydrauliccylinder, caused initially principally by the weight of the piston, ramand plug, is relieved through the relief valve 96 which returnshydraulic fluid into the upper chamber 24. By using this feedbackconnection, a higher flow rate is achievable and thereby faster pistonoperation for a given source pressure is attainable. The check valve 94prevents fluid from entering the port of the valve 87.

When the limit switch 79 is struck by the actuator 76 (see FIG. 1) asignal is provided by the electric control system to operate the valve87 to the neutral or closed position and to operate the valve 98 to acondition wherein the port is connected to the port 99 and wherein theport 101 is connected to the port 102. Hydraulic fluid from the source93 then flows through the regulator valve 103 and the pressure reducingvalve 104, through the ports 100 and 99 into the upper chamber 24. Inthis condition, the lower chamber 26 is exhausted through the port 101and the port 102. The piston thereby continues to move down at a reducedrate of speed until it encounters resistance due to solidified material.The movement of the piston and the pressure in the upper chamber 24 maybe compared in FIG. 5 wherein the displacement curve represents theposition of the piston and the pressure curve represents the pressure inthe upper chamber as sensed by a pressure sensing device 106 (see FIG.4). Rapid movement of the piston downwardly continues along the dottedline to the position d,. It is then'slowed down by the action caused bythe limit switch 79 as it moves from position d, to the position d Allthis occurs in the time interval t,,t, and wherein the pressure in theupper chamber is at its minimum level.

Once the plug engages the top of the ingot, and encounters resistancedue to partially solidified material, movement of the plug is halted inthe position d The pressure in the upper chamber 24 also remainsconstant as fluid continues to flow into the chamber 24. This isbecause, during the displacement of the piston from d, to d the pistonmoves downwardly with the flow of fluid out of the lower chamber 26 andis not forced down by increasing pressure.

When the fluid fills the upper chamber 24, the pressure begins toincrease. At the time t or shortly thereafter, the increased forceresulting from the increase in pressure causes hot working in the metaland the plug and piston then begin to move down. This pressurebuiidup-incre ases'and downward piston movement continues inthe intervaltz-tg; until the pressure in the upper chamber 24 exceeds the sensinglevel of the pressure sensor 106. When this occurs,'the electric,

control system operates the valve 98 to a neutral or off condition andoperates the valve 87 to a condition indicated by the crossed arrows,that is, with the port 89 connected to the port 90 and with the port 88connected to the port 91. In this condition, hydraulic fluid underpressure from the source 93 flows through the line 92, the ports 89 and90 and the check valve 94 into the lower chamber 26. At the same time,hydraulic fluid in the upper chamber 24 is exhausted through the ports88 and 91. As a result of switching to the exhaust condition, it may beseen that the pressure in the upper chamber 24 at time t;, begins todrop quickly and that the piston 23 begins to move upwardly from itslowermost position at d Such movement continues until the actuator plate76 engages the limit switch 77.'At this time, the valve 87 is operatedto the originally described condition, and the valve 98 is operated tothe neutral or off condition. Downward movement of the piston thenresumes.

Generally speaking, and for most materials, satisfactory results areobtained when the plug is held in contact with the ingot for a timeinterval in the range of about 3 to seconds, and wherein it is held outof contact with the ingot for a time interval in the range of 2 to 6seconds. In order to achieve this, the electronic control systemincorporates time delay circuitry to provide a desired dwell time aftertriggering of the upper limit switch 37 or of the pressure sensor 28.Moreover, in order to achieve freedom from segregation for mostmaterials, the size of the plug and the cooling thereof are chosen sothat the heat removal from the central section of the ingot is such thatthe diameter of the central solidified region formed by the coolant plugis not greater than about 8 inches smaller than the effective diameterof the ingot. This prevents the annular molten pool (81 in FIG. 2) frombecoming excessively deep whereby slower solidification time wouldresult with consequent segregation problems. Immersion times of lessthan about 3 seconds may result in the solidification of a thin tightlyadhering shell on the plug which proceeds to build up on succeedingimmersions. Moreover, sufficient upward force must be provided as toenable the plug to disengage from the ingot after the plunge and dwell.Immersion times greater than about 10 seconds make it extremelydifficult to remove the plug from the ingot. Generally speaking, adisengagement time of about 2 to 6 seconds enables molten material torun back into the cavity produced by the plug before the plug is broughtback into contact with the ingot.

For most materials the pressure exerted by the plug should exceed about60 psi. At pressures less than this, there may be a tendency forsolidified metal to adhere to the plug as the plug is withdrawn. Thisoccurs because insufficient hot working has been effected to weld thelayers immediately adjacent the plug to the lower layers of thesolidified central region of the ingot. The minimum required pressurevaries depending on the properties of the material, however, and somematerials may require a higher pressure. In particular, those materialswhich have a solidification range of about 50 or more, and thosematerials which have low thermal conductivity, such as 304 stainless,typically require much higher pressure levels to insure proper ingotformation. There is no theoretical limit to the maximum pressure, but itshould not, of course, exceed the loading capabilities of the ingotpulling apparatus.

Use of the invention in connection with Fe26Cr, FeZOCr, material soldunder the trademark WASPAL- LOY, and 4340 steel, have resulted in ingotswherein longitudinal grains were formed in the annular regionsurrounding the central region of the ingot and wherein the centralregion consisted of substantially smaller grains of an equiaxial nature.These smaller grains .in the central region of the ingot have a randomorientation with respect to those in the annular region where the longergrains are present. Experiments have shown that the presence ofsegregation problems in ingots cast in accordance with the invention aresubstantially eliminated, and very dramatic improvement results inconnection with large diameter in ots, such as those ingots exceedingabout 12 inches efective diameter. By effective diameter, it is meantthe diameter of a cylindrical ingot or the distance across the flats ofa round comer square ingot.

The dramatic improvement of ingots cast according to the invention interms of the elimination of segregation problems has been observed inconnection with 4340 steel. Ingots cast without central cooling inaccordance with the invention have been compared with ingots cast inaccordance with the invention and have been found to be of significantlylower ductility. For example, tests of 4340 steel show an improvementofalmost 10 percentage points in the percent reduction in area values overvacuum cast ingots in which central cooling was not employed.

It may therefore be seen that the invention provides an improved methodand apparatus for continuous casting in which problems of segregationare minimized. The invention has particular advantage in connection withthe vacuum casting of large diameter ingots and enables a significantimprovement in the casting rates attainable.

Various modifications of the invention in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the appendant claims.

What is claimed is:

1. Apparatus for use in a continuous casting system including a castingmold for continuously casting an ingot of solid cross section and meansfor pulling the cast ingot downwardly through the mold, said apparatuscomprising, a plug having a solid cross section less than the crosssection of the casting mold, means for supporting said plug above thecasting mold aligned substantially with the longitudinal axis thereof,means for moving said "plug reciprocally to repeatedly engage anddisengage the central region of the molten pool at the top of the ingotin the casting mold, and means for removing heat from said plug to coolthe ingot being cast in the mold in the central region thereof.

2. Apparatus according to claim 1 wherein the lower end of said plug hasa conical surface.

3. Apparatus according to claim 1 wherein said heat removing meanscomprise means for circulating a fluid coolant in contact with saidplug.

4. Apparatus according to claim 1 wherein said moving means include areciprocable hydraulic cylinder, means for sensing a rise' in pressurein the upper part of said cylinder to a predetermined level in responseto resistance to movement of said plug exerted by the ingot, and meansresponsive to the sensing of the predetermined pressure for stoppingsaid plug and for reversing the action of said cylinder after apredetermined time delay.

5. Apparatus according to claim 4 wherein said moving means include alimit switch positioned to be actuated upon a rise of said plug to apredetermined upper position, and means responsive to the actuation ofsaid limit switch for stopping said plug and for reversing the action ofsaid cylinder after a predetermined time delay.

1. Apparatus for use in a continuous casting system including a castingmold for continuously casting an ingot of solid cross section and meansfor pulling the cast ingot downwardly through the mold, said apparatuscomprising, a plug having a solid cross section less than the crosssection of the casting mold, means for supporting said plug above thecasting mold aligned substantially with the longitudinal axis thereof,means for moving said plug reciprocally to repeatedly engage anddisengage the central region of the molten pool at the top of the ingotin the casting mold, and means for removing heat from said plug to coolthe ingot being cast in the mold in the central region thereof. 2.Apparatus according to claim 1 wherein the lower end of said plug has aconical surface.
 3. Apparatus according to claim 1 wherein said heatremoving means comprise means for circulating a fluid coolant in contactwith said plug.
 4. Apparatus according to claim 1 wherein said movingmeans include a reciprocable hydraulic cylinder, means for sensing arise in pressure in the upper part of said cylinder to a predeterminedlevel in response to resistance to movement of said plug exerted by theingot, and means responsive to the sensing of the predetermined pressurefor stopping said plug and for reversing the action of said cylinderafter a predetermined time delay.
 5. Apparatus according to claim 4wherein said moving means include a limit switch positioned to beactuated upon a rise of said plug to a predetermined upper position, andmeans responsive to the actuation of said limit switch for stopping saidplug and for reversing the action of said cylinder after a predeterminedtime delay.